Four new Planothidium species (Achnanthidiaceae, Bacillariophyceae) from the Karst region of Guizhou in China

$¢PhytoKeys

PhytoKeys 259: 81-102 (2025)
DOI: 10.3897/phytokeys.259.150757

Research Article

Four new Planothidium species (Achnanthidiaceae,
Bacillariophyceae) from the Karst region of Guizhou in China

Yun Li'®, Lin-Xin Lu'®, Hui-Wen Zhou'™®, Pan Yu2®, John Patrick Kociolek?®, Wan-Ting Pang'™,
Quan-Xi Wang™®, Qing-Min You'®

1 College of Life Sciences, Shanghai Normal University, Shanghai 200234, China

2 School of Environment and Geographical Sciences, Shanghai Normal University, Shanghai 200234, China

3 Museum of Natural History and Department of Ecology and Evolutionary Biology, University of Colorado, UCB 218, Boulder CO80309, USA
Corresponding author: Qing-Min You (yougm1117@shnu.edu.cn)

OPEN Qaceess

Academic editor: Kalina Manoylov
Received: 20 February 2025
Accepted: 20 May 2025

Published: 25 June 2025

Citation: Li Y, Lu L-X, Zhou H-W, Yu P
Kociolek JP Pang W-T, Wang Q-X, You
Q-M (2025) Four new Planothidium
species (Achnanthidiaceae,
Bacillariophyceae) from the Karst
region of Guizhou in China. PhytoKeys
209: 81-102. https://doi.org/10.3897/
phytokeys.259.150757

Copyright: © Yun Li et al.
This is an open access article distributed under
terms of the Creative Commons Attribution

License (Attribution 4.0 International - CC BY 4.0).

Abstract

Four new species of monoraphid diatoms belonging to the genus Planothidium were
studied and described from the karst region of Guizhou Province, China. The morpho-
logical characteristics of these new species were observed and documented using light
and scanning electron microscopy. Planothidium pseudoreichardtii sp. nov. exhibits a
broad sinus at the central area on the interior of the rapheless valve, linear shallow de-
pressions and a unique enlarged areola at the end of the striae on the exterior rapheless
valve. Planothidium liboensis sp. nov. is characterized by 4-5 rows striae on both valves,
a cavum at the central area on the interior rapheless valve and irregular depressions on
the exterior rapheless valve. Planothidium angustirostratum sp. nov. features elliptical
valves with narrowly rostrate apices, a cavum at the central area of interior rapheless
valve and circular depressions on the exterior rapheless valve. Planothidium maolanen-
sis sp. nov. has elliptical valves without protracted apices, coarsely-spaced striae on the
one side of the central area of the both valves. These four new species were compared
with other similar Planothidium taxa. In addition, ecological information was provided
and the stability of some features was discussed.

Key words: Diatoms, karst, monoraphid, new taxa, Planothidium, taxonomy

Introduction

Monoraphid diatoms are particularly intriguing due to their structural asymmetry,
characterized by the presence of a raphe on one valve and its absence on the oth-
er (Kulikovskiy et al. 2016). The genus Planothidium Round & Bukhtiyarova was
established to accommodate monoraphid diatoms previously classified within
the genus Achnanthidium Kitzing (Round and Bukhtiyarova 1996). Currently, ap-
proximately 129 accepted taxa of Planothidium are recorded in AlgaeBase (Gui-
ry and Guiry 2024). The genus is characterized by the presence of multiseriate
striae on both the raphe valve (RV) and rapheless valve (SV), as well as asym-
metrical structures in the central area of the rapheless valve in many species
(Morales 2006). Since its establishment, Planothidium has undergone signifi-
cant taxonomic refinement, with studies describing new species and exploring

81

Yun Li et al.: Four new Planothidium species

its morphological and ecological diversity. The application of scanning electron
microscopy and molecular techniques has further enhanced the differentiation
of Planothidium species and deepened our understanding of their morphological
features (Novis et al. 2012; Alvarez-Blanco and Blanco 2013; Van De Vijver et al.
2013; Jahn et al. 2017; Van De Vijver and Bosak 2019; Wetzel et al. 2019).

Morphological studies of Planothidium have focused on key characteris-
tics, such as the structure of the rapheless valve's central area, valve outline,
and striae patterns. Morales (2006) proposed a classification framework for
Planothidium species based on the central area morphology of the rapheless
valve, dividing them into four groups: (1) species with continuous, uninterrupt-
ed striae in the central area, such as P. daui (Foged) Lange-Bertalot; (2) spe-
cies with a variable, clear space in the central area that does not show any
indentation or convexity, such as P minutissimum (Krasske) Lange-Bertalot;
(3) species with a depressed central area without striae, such as P. lanceolatum
(Brébisson ex Kiitzing) Lange-Bertalot; (4) species with a depressed region on
one side of the central area, which is capped to form a cave-like structure called
a “cavum”, such as P. biporomum (Hohn & Hellerman) Lange-Bertalot (Morales
2006). In the study by Stancheva et al. (2020), Planothidium species were cate-
gorized into three groups: those with a sinus (single horse-shoe shaped mark),
such as P lanceolatum; those with a cavum (double horse-shoe shaped mark),
such as P. frequentissimum (Lange-Bertalot) Lange-Bertalot; and those lacking
both a sinus and a cavum, such as P. minutissimum (Krasske) Morales and
P. daui (Foged) Lange-Bertalot. Moreover, beyond classic morphological mea-
surements (such as width, length, length-to-width ratio, and number of striae
per 10 um), several other features observed in the SEM have been suggested
as useful for species identification, including (1) the pattern of surface smooth-
ness on the rapheless valve; (2) the number of areolae per stria on the raph-
eless valve; and (3) whether the striae on the rapheless valve interrupt at the
valve margin, thus influencing the number of areolae at the valve edge (Wetzel
et al. 2019). In addition to morphological studies, molecular tools have provid-
ed new insights into the taxonomy and phylogeny of Planothidium. Molecular
research has confirmed the presence of two distinct clades within the genus,
one with species possessing a sinus or cavum in the rapheless valve and the
other lacking these structures (Jahn et al. 2017).

Currently, research on the genus Planothidium in China remains limited. In
marine environments, one new species has been reported and described (Li
et al. 2024). However, in freshwater ecosystems, only a few species have been
recorded as new records, and no new species have been reported so far (Liu et
al. 2015, 2016; Wang et al. 2019). Karst ecosystems, characterized by carbon-
ate-rich and nutrient-poor waters, remain underexplored. These unique habi-
tats may drive morphological differentiation and adaptive evolution, offering
immense potential for diatom taxonomy studies. Recently, several new diatom
species have been described from karst regions, such as Sellaphora gologonica
Lai, Ector & Wetzel, Germainiella legionensis Blanco, Borrego-Ramos & Olenici,
Achnanthidium mediolanceolatum Yu, You & Kociolek, Amphora baotuensis Li,
Nagumo & Xu, and Fallacia cinariana Somek, Hamilton, Solak, Beauger & Sevin-
dik et al. The discovery of these species highlights the underexplored diatom
biodiversity in karst environments (Borrego-Ramos et al. 2018; Lai et al. 2018;
You et al. 2019b; Li et al. 2022; Somek et al. 2025).

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 99

Yun Li et al.: Four new Planothidium species

In recent years, we carried extensive surveys of diatoms species diversity
of China (Kociolek et al. 2016b, 2016a, 2019; Lowe et al. 2017; You et al. 2017,
2019a; Yu et al. 2017; Zhou et al. 2024). Based on the collections developed for
these studies, four new Planothidium species within karst landform of Guizhou
have been discovered. This study provides detailed descriptions, as well as light
and scanning electron microscopy of these new species: Planothidium pseudore-
ichardtii sp. nov., Planothidium liboensis sp. nov., Planothidium angustirostratum
sp. nov. and Planothidium maolanensis sp. nov., comparing them with similar
species within the genus. This work contributes valuable reference material for
the taxonomy and morphology of freshwater Planothidium, while emphasizing
the importance of karst habitats as biodiversity hotspots for diatoms.

Materials and methods

Diatom samples were collected from Xiaogikong Scenic Area and Maolan
Nature Reserve (Libo County, Guizhou Province), which is characterized by a
subtropical climate and is a typical karst landscape. The sampling sites were
distributed in various water bodies, including streams, ponds and underground
rivers. The water parameters (pH, Temperature and Conductivity) were mea-
sured using a YSI Pro Plus multiparameter meter (YSI, Ohio, USA). Samples
were collected using tweezers and a knife, then placed into sealed plastic bot-
tles with the addition of formalin for preservation (final concentration of 4%).
Information about the sampling sites is listed in Table 1.

In the laboratory, samples were processed to remove organic matter and
excess impurities using a microwave-assisted reaction system (Model MARS,
CEM Corporation, Charlotte, USA) with concentrated nitric acid. The specific
steps for sample processing and methods of preservation are as described in
You et al. (2021). Clean samples after processing were prepared for both light
microscopy (LM) and scanning electron microscopy (SEM). For LM, samples
were encapsulated in Naphrax and observed using an Olympus BX-53 micro-
scope with DIC optics and a 1.4 numerical aperture, 100~ oil immersion objec-
tive. For SEM, the same samples were dried on metal stubs and imaged using

Table 1. Locality data and habitat for samples studied.

No. of samples

GZ201510041P

GZ201510045

GZ201510051

GZ201510066

GZ201510099

GZ201510100

GZ201510108

Location
Xiaoqikong
Scenic Area
Xiaogikong
Scenic Area
Xiaoqikong
Scenic Area
Xiaoqikong
Scenic Area

Maolan Nature

Reserve

Maolan Nature

Reserve

Maolan Nature

Reserve

' ; Altitude Water Cond. Collection
Coordinates Habitat (m) Temp (°C) pH (uS/em) Collector Date
25°15'41"N, | Attached to rocks in 780 18.0 ZB 226 Kociolek & | 10.2.2015
107°45'19"E the pond Wang Q.X.
25°15'36'N, | Attached tothe stones | 780 18.0 io | 226 Wang Q.X. & | 10.2.2015
107°45'16"E sin the rushing water Kociolek J.P.
25°15'36'N, | Attached to floating 780 18.0 7,5) 226 Wang Q.X. & | 10.2.2015
107°45'16"E things in the pond Kociolek J.P.
25°15'02'N, Attached to rocks 629 19.5 8:0:| -215 Wang Q.X. & | 10.2.2015
107°42'46"E beneath the waterfall Kociolek J.P.
25°17'32'N, | Floating in the slow 650 18.0 7.9 203 Wang Q.X. & | 10.4.2015
108°04'16"E subsurface stream Kociolek J.P.
25°17'32'N, | Floating in the slow 650 18.0 ZO) 203 Wang Q.X. & | 10.4.2015
108°04'16"E subsurface stream Kociolek J.P.
25°17'35'N, | Attached to rocks ina 811 18.0 7.8) 205 Wang Q.X.& | 10.4.2015
108°04'42"E rapid river. Kociolek J.P.

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757

83

Yun Li et al.: Four new Planothidium species

a SU8010 SEM at 2 kV with a working distance (WD) of less than 6 mm (Hitachi
High-Technologies Corp., Tokyo, Japan). To determine the relative abundance
of the new species and record co-occurring diatom species, 400 intact and
identifiable diatom valves from each sample were identified under light micros-
copy (1000x). The images were compiled using Adobe Photoshop 2023. The
morphological terminology follows (Morales 2006; Van De Vijver et al. 2018;
Wetzel et al. 2019; Morais et al. 2020). The holotype images of each species
correspond to specimens that have been circled on the permanent slides. All
samples and permanent slides are stored at Lab of Algae and Environment,
College of Life Sciences, Shanghai Normal University.

Results

Phylum Bacillariophyta

Class Bacillariophyceae Haeckel, 1878

Order Achnanthales Silva, 1962

Family Achnanthidiaceae Mann, 1990

Genus Planothidium Round & Bukhtiyarova, 1996

Planothidium pseudoreichardtii Q-M. You, P. Yu & J.P. Kociolek, sp. nov.
Figs 1, 2

Holotype. SHTU! Slide GZ201510045, holotype illustrated in Fig. 1C, J. Diatom
samples are housed in the Lab of Algae and Environment, College of Life Sci-
ences, Shanghai Normal University, China.

Isotype. COLO! Material 11111, Slides are housed in the Kociolek Collection, Uni-
versity of Colorado, Museum of Natural History Diatom Herbarium, Boulder, U.S.A.

Type locality. CHINA. Xiaogikong Scenic Area, Libo County, Guizhou Province,
25°15'36'N, 107°45'16'E, altitude: 780 m, collected by Kociolek J.P. & Wang
Q.X., on October 2, 2015.

Description. Light microscopy (LM) (Fig. 1A—P). Valves elliptical with slight-
ly constricted ends and rostrate extensions. Valve dimensions (n = 60): Length
11.5-19.0 um, width 5.5—7.5 um. Rapheless Valve (Fig. 1A—H): Axial area narrow,
straight, linear. Central area features a U-shaped hyaline region widening towards
the valve margin, on the opposite side, striae extend to the axial area. Striae radi-
ate along the valve outline, 14-17 in 10 um (measured opposite hyaline area). Ra-
phe Valve (Fig. 1I-P): Axial area narrow, straight, linear. Central area rectangular
to nearly circular, bordered by 3-4 shortened, asymmetrical striae on each side.
Raphe branches straight. Striae radiate along the valve outline, 14-18 in 10 um.

Scanning electron microscopy (SEM) (Figs 1Q, R, 2A, B). Rapheless Valve
(Fig. 1Q, R): Striae composed of 3-4 rows of circular areolae, the outer two
rows larger than those near the center. Striae narrow to 2 rows towards the
axial area and may expand to 3-4 rows near the valve margin, a larger areo-
la marks the end of striae near the valve margin (Fig. 1Q, white arrow). The
central and axial areas feature irregular, shallow, linear depression. Internally,
areolae are covered. A clearly visible sinus is present on one side of the central
area, forming a shallow circular depression (Fig. 1R, white arrow). Raphe Valve
(Fig. 2A, B): Striae composed of 3-4 rows of circular areolae. Striae narrow
towards one end near the axial area. The central area typically displays 2 to 3

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 34

Yun Li et al.: Four new Planothidium species

|

-
~ _
-
, *
~
7
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*

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*:

Figure 1. Planothidium pseudoreichardtii sp. nov. LM; A-H. Rapheless valves; I-P. Raphe valves. “=” indicates the dif-

ferent valves of the same frustule. Q. SEM external view of an entire rapheless valve. The white arrow points to a larger
areola. R. SEM internal view of an entire rapheless valve, showing a distinct sinus in the central area (white arrow). Scale

bar as shown. Scale bars: 10 um (A-P).

shortened striae. Striae almost extend onto the valve margin. Raphe branches
are Straight, with proximal raphe endings expanding into pores, surrounded by
shallow, drop-like depressions. Distal raphe ends are curved in the same direc-
tion, briefly extending onto the mantle. Internally, proximal raphe endings are
slightly deflected to the opposite side, distal raphe endings terminate in a small
helictoglossa. Central nodule is raised, with striae wider raised virgae and sunk-

en between them, and areolae are individually covered.

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 95

Yun Li et al.: Four new Planothidium species

; A _
va : , Bak i!

Figure 2. Planothidium pseudoreichardtii sp. nov. SEM; A. External view of an entire raphe valve. B. Internal view of an
entire raphe valve, scale bar as shown. Scale bar as shown.

Etymology. The species was named for its outline being similar to P reichardtii.

PhycoBank registration. http://phycobank.org/105527

Distribution and ecology. This species has currently only been found at its type
locality, where it was collected from rocks in a rapid stream (water temperature
18.0 °C, elevation 780 m). The species was observed in sample GZ201510041P
(2.0%), GZ201510045 (2.5%). In the type material (GZ201510045), P pseudore-
ichardtii was rare. The associated diatom flora included Navicula lundii Reichardt
(19.5%), Achnanthidium delmontii Pérés, le Cohu & Barthés (12.0%), Planothid-
ium rostratum (@strup) Lange-Bertalot (6.0%), Platessa hustedtii Lange-Bertalot
(5.5%), and Achnanthidium minutissimum (Kitzing) Czarnecki (5.0%), and other
taxa with lower abundance (less than 5%) such as Navicula capitatoradiata Ger-
main, Achnanthidium rivulare Potapova & Ponader, Staurosira construens Ehren-
berg, Punctastriata nyingchiensis Luo & Wang, Cocconeis placentula Ehrenberg,

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 86

Yun Li et al.: Four new Planothidium species

and Nitzschia palea Smith. Additional ecological information is presented in
Table 1.

Remarks. Due to their similar valve outlines, Planothidium pseudoreichardtii
sp. nov. and Planothidium liboensis sp. nov. were compared with several mor-
phologically similar taxa, including Planothidium reichardtii Lange-Bertalot
& Werum, Planothidium rostratum (@strup) Lange-Bertalot and Planothidium
xinguense Morais, Wetzel & Bicudo (Table 2). However, P pseudoreichardtii
sp. nov. can be easily distinguished from P liboensis sp. nov. because the
former possesses a sinus, while the latter has a cavum on the center of the
rapheless valve. Meanwhile, P. pseudoreichardtii can be differentiated from
P reichardtii by several distinct features: (1) in LM, the central area of the raph-
eless valve in P pseudoreichardtii has a broader hyaline area outlined by 3-5
striae, whereas in P reichardtii, the hyaline area is outlined by only 1-2 striae;
(2) The striae on the rapheless valve of P pseudoreichardtii consist of 3-4
rows of circular areolae, with the outermost two rows being larger than the
central ones, whereas P reichardtii has uniformly sized areolae in its striae;
(3) the striae of P pseudoreichardtii terminates at the valve margin with a dis-
tinctly enlarged areola, while P. reichardtii doesn’t have it. (4) irregular shallow
slit-like depressions are present in the central and axial areas of the rapheless
valve in P pseudoreicharatii, whereas no depressions were observed on the
rapheless valve of P reichardtii.

Table 2. Comparison of morphological characteristics of Planothidium pseudoreichardtii sp. nov. & Planothidium liboen-
sis sp. nov. and closely related taxa.

so tel gir .. Planothidium reichardtii Planothidium Rlpnounolum Planothidium xinguense
pseudoreichardtii ; : rostratum (@strup) :
Lange-Bertalot & Werum | liboensis sp. nov. Morais
Sp. nov. Lange-Bertalot
Reference This study Werum and Lange- This study Lange-Bertalot 1999; Morais et al. 2020
Bertalot 2004 Weizel et al. 2019

Valve outline Elliptical Elliptical to linear-elliptical Elliptical Elliptical Elliptical, asymmetrical
Apices Rostrate Subrostrate to capitate Rostrate Short to long rostrate | Short to long rostrate
Length 11.5-19.0 um 8-18 um 12-18 pm 6.5-15.0 um 12-14 um
Width 55-7.5;m 4.0-6.5 um 5-7 um 4.0-6.5 um 6.7-8.2 um
Central area (RV) Rectangular to Nd Rectangular to | Rectangular to slightly | Irregular, rectangular to

nearly circular nearly circular round slightly rounded
Axial area (RV) Narrow linear linear Narrow linear Narrow, linear, Narrow linear

widening towards the
central area
Striae (RV) 14-18/10 um Nd 14-17/10 um 12-16/10 pm 12-14 um
Areolae composition | 3-4 rows areolae Nd 4-5 rows areolae 3-4rows areolae 3-4 rows areolae
of striae (RV)
Axial area (SV) Narrow, linear to linear Narrow linear | Narrow, straight, linear | Narrow, linear, expanded
lanceolate in the central area
opposite to the cavum

Additional structure Sinus Sinus Cavum Cavum Cavum
(SV)
Striae (SV) 14-17/10 um Nd 15-16/10 um 12-14/10 pm 12-14 um
Areolae composition | 3-4 rows areolae Nd 4-5 rows areolae 3-4 rows areolae 1-3 rows areolae
of striae (SV)

Note: “Nd” indicates no data available in the reference.

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 97

Yun Li et al.: Four new Planothidium species

-

Figure 3. Planothidium liboensis sp. nov. LM; A-H. Rapheless valves; I-P. Raphe valves.

‘ . , = : 4
~ = y = A ie =
= 2 se 3 Rg ee —— wa ‘ ar
F LX SO, ea er Pott Sri

Planothidium liboensis Q-M. You, P. Yu & J.P. Kociolek, sp. nov.
Figs 3,4

Holotype. SHTU! Slide GZ201510051, holotype illustrated in Fig. 3F, N. Diatom
samples are housed in the Lab of Algae and Environment, College of Life Sci-
ences, Shanghai Normal University, China.

Isotype. COLO! Material 11117, Slides are housed in the Kociolek Collec-
tion, University of Colorado, Museum of Natural History Diatom Herbarium,
Boulder, U.S.A.

Type locality. CHINA. Xiaogikong Scenic Area, Libo County, Guizhou Province,
25°15'36'"N, 107°45'16'E, altitude: 780 m, collected by Wang Q.X. & Kociolek
J.P.,on October 2, 2015.

Description. Light microscopy (LM) (Fig. 3A—P). Valves elliptical with slightly
constricted ends and rostrate extensions. Valve dimensions (n = 75): Length
12-18 um, width 5-7 um. Rapheless Valve (SV) (Fig. 3A—H): Axial area narrow,
straight, linear. Central area with a large, unilateral, horseshoe-shaped hyaline
area containing a clearly visible cavum, on the opposite side, striae extend to
the axial area. Striae radiate along the valve outline, 15-16 in 10 um (measured
opposite the cavum). Raphe Valve (RV) (Fig. 3I-P): Axial area narrow, straight,
and linear, widening slightly towards the central area. Central area irregular, rect-
angular to slightly rounded, bordered by 2—3 shortened, asymmetrical striae on

indicates the different valves

any
—
=

of the same frustule. Q. SEM external view of an entire rapheless valve, scale bar as shown. Scale bars: 10 um (A-P).

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 39

Yun Li et al.: Four new Planothidium species

Figure 4. Planothidium liboensis sp. nov SEM; A. Internal view of an entire rapheless valve; B. External view of an entire
raphe valve; C. Internal view of an entire raphe valve. Scale bar as shown.

each side. Raphe branches straight, with proximal raphe endings drop-like. Stri-
ae radiate along the valve outline, 14-17 in 10 um.

Scanning electron microscopy (SEM) (Figs 3Q, 4A-C). Rapheless Valve
(Figs 3Q, 4A): Striae composed of 4 to 5 rows of circular areolae, typically par-
allel to each other. Near the central area, striae often narrow to 3 areolae adja-
cent to the axial area. Striae extend onto the valve margin, and no areolae exist
on the mantle. The axial area and central area exhibit irregular depressions.
The cavum opening is broad, striae are distinctly wider than the virgae and

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 89

Yun Li et al.: Four new Planothidium species

sunken between them, areolae are covered by individual hymenes. Raphe Valve
(Fig. 4B, C): Striae composed of 4 to 5 rows of circular areolae, with some striae
consisting of 3 areolae near the axial area and valve margin. Striae extend onto
the valve margin, and no areolae exist on the mantle. The central area typically
displays 2 to 3 shortened areolae. Raphe branches are straight, with proximal
raphe endings expanding into pores, surrounded by shallow, drop-like depres-
sions. Distal raphe ends are curved in the same direction, briefly extending onto
the mantle. Internally, proximal raphe endings are slightly deflected to the op-
posite side, distal raphe endings terminate in a small helictoglossa. The central
nodule is raised, striae are markedly wider than the virgae and sunken between
them, and areolae are covered by individual hymenes.

Etymology. The species was named for the type locality, Libo County.

PhycoBank registration. http://phycobank.org/105528

Distribution and ecology. This species has currently only been found at its
type locality, where it was collected from rocks and floating debris in ponds,
as well as from rocks beneath a waterfall (water temperature 18.0-19.5 °C,
elevation 629-780 m). The species was observed in samples GZ201510045
(1.0%), GZ201510051 (5.8%), and GZ201510066 (1.5%). In the type material
(GZ201510051), P liboensis exhibited a relatively high abundance. The asso-
ciated diatom flora included Sellaphora sp. (15.5%), Planothidium rostratum
(12.5%), Fallacia sp. (5.0%), Achnanthidium minutissimum (5.0%), Nitzschia
palea (5.0%), and other taxa with lower abundance (less than 5%) such as Pla-
nothidium ellipticum (Cleve) Round & Bukhtiyarova, Sellaphora rotunda Wetzel,
Ector, Van de Vijver, Compere & Mann, Achnanthidium spp., Punctastriata nying-
chiensis, Staurosira construens, Cocconeis placentula, and Platessa hustedtii.
Additional ecological information is presented in Table 1.

Remarks. In LM, P liboensis, P rostratum, and P. xinguense exhibit broadly sim-
ilar valve outlines, and all display a cavum. However, P liboensis can be distin-
guished from other taxa by the following features: (1) P. liboensis has a denser
arrangement of striae, typically consisting of 4-5 rows of similarly-sized round
areolae, and the striae are distinctly wider than the virgae. In contrast, the striae of
P rostratum in the rapheless valve consist of 3-4 rows of areolae, where the two
middle rows are smaller than the outer rows. The striae of P xinguense in raph-
eless valve are composed of 1-3 rows of areolae; (2) The central area of raph-
eless valve in P liboensis exhibits distinctive irregularly shaped shallow depres-
sions, whereas P rostratum features shallow, slit-like depressions. P xinguense
also exhibits irregularly shaped shallow depressions in the central area, but their
shape differs from those in P liboensis. Additionally, P xinguense has slit-like de-
pressions in the axial area; (3) compared to P rostratum, the cavum of P liboensis
has a more open aperture; (4) compared to P. xinguense, P. liboensis exhibits
weaker striae radiation and a narrower central area on the rapheless valve.

Planothidium angustirostratum Q-M. You, P. Yu & J.P. Kociolek, sp. nov.
Figs 5, 6

Holotype. SHTU! Slide GZ201510051, holotype illustrated in Fig. 5F, N. Diatom
samples are housed in the Lab of Algae and Environment, College of Life Sci-
ences, Shanghai Normal University, China.

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 90

Yun Li et al.: Four new Planothidium species

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=
—

Figure 5. Planothidium angustirostratum sp. nov. LM; A-H. Rapheless valves; I-P. Raphe valves. “=” indicates the different
valves of the same frustule. Q. SEM external view of an entire rapheless valve, scale bar as shown. Scale bars: 10 um (A-P).

Isotype. COLO! Material 11117, Slides are housed in the Kociolek Collection, Uni-
versity of Colorado, Museum of Natural History Diatom Herbarium, Boulder, U.S.A.

Type locality. CHINA. Xiaogikong Scenic Area, Libo County, Guizhou Province,
25°15'36'"N, 107°45'16'E, altitude: 780 m, collected by Wang Q.X. & Kociolek
J.P., on October 2, 2015.

Description. Light microscopy (LM) (Fig. 5A-P). Valves elliptic-lanceolate
with narrowly rostrate to subcapitate apices. Valve dimensions (n = 40): Valve
length 17-21 um, width 6.5-7.0 um. Rapheless Valve (Fig. 54—H): Axial area
narrow, linear, slightly broadened in the middle. A horseshoe-shaped hyaline
area present on one side of the central area, containing a cavum. Striae weakly
radiate along the valve outline, 13-14 in 10 um (measured opposite the hyaline
area). Raphe Valve (Fig. 5I-P): Axial area narrow, straight, linear. Central area
subcircular to rectangular, bordered by 3-4 slightly shortened striae on each
side. Raphe branches straight, with proximal raphe endings drop-like. Striae ra-
diate along the valve outline, 13-14 in 10 um.

Scanning electron microscopy (SEM) (Figs 5Q, 6A-C). Rapheless Valve
(Figs 5Q, 6A): Striae composed of 3-4 rows of circular areolae, narrowing near
the central area adjacent to the axial area. Striae extend onto the valve mantle.
Irregular circular depressions present between the axial area, central area, even

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 91

Yun Li et al.: Four new Planothidium species

s / Px aN ee es: atte
Figure 6. Planothidium angustirostratum sp. nov. SEM; A. Internal view of an entire rapheless valve; B. External view of an
entire raphe valve; C. Internal view of an entire raphe valve. Scale bar as shown.

between striae. Internally, areolae covered individually, striae wider than virgae
and sunken between them. Cavum opening slightly constricted. Raphe Valve
(Fig. 6B, C): Striae composed of 3-4 rows of areolae, striae near the central
area regularly shortened and narrowing towards the axial area. Striae extend
onto the valve mantle. Raphe branches straight, with proximal raphe endings
expanding into pores surrounded by drop-like depressions. Distal raphe ends
are curved in the same direction, slightly extending onto the valve mantle. Inter-
nally, proximal raphe endings slightly deflected to the opposite side, and distal
raphe endings terminating in a faint helictoglossa. Internally, areolae covered
individually, striae wider than virgae and sunken between them.

Etymology. The species was named for its narrowly rostrate valve apices.

PhycoBank registration. http://phycobank.org/105529

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 99

Yun Li et al.: Four new Planothidium species

Distribution and ecology. This species has currently only been found at its type
locality, where it was collected from floating debris in a pond (water temperature
18.0 °C, elevation 780 m). The species was observed only insample (GZ201510051)
at a low relative abundance (0.75%). In the type material, P angustirostratum was
rare. The associated diatom flora included Sellaphora sp. (15.5%), Planothidium ros-
tratum (12.5%), Planothidium liboensis (5.75%), Fallacia sp. (5.0%), Achnanthidium
minutissimum (5.0%), Nitzschia palea (5.0%), and other taxa with lower abundance
(less than 5%) such as Planothidium ellipticum, Sellaphora rotunda, Achnanthidium
spp., Punctastriata nyingchiensis, Staurosira construens, Cocconeis placentula, and
Platessa hustedtii. Additional ecological information is presented in Table 1.

Remarks. Due to the broadly similar valve outline in LM and the presence of a
cavum, Planothidium angustirostratum sp. nov. can be compared with Planothidium
brasiliense Wetzel & Blanco, Planothidium potapovae Wetzel & Ector and Planothid-
ium rostratoholarcticum Lange-Bertalot & Bak (Table 3). In addition to differences
in striae density and valve length and width, Planothidium angustirostratum sp. nov.
also can be distinguished from these similar species by the following features:
(1) with narrower rostrate to subcapitate apices and a narrower cavum aperture;
(2) a smaller central area, ranging from rectangular to nearly circular; (3) its striae
denser on the raphe valve, consist of 3-4 rows of circular areolae; (4) circular shal-
low depressions are present in the central area, axial area, and between the striae,
unlike the slit-like depressions observed in P potapovae and P rostratoholarcticum.

Table 3. Comparison of morphological characteristics of Planothidium angustirostratum sp. nov. and closely related taxa.

Planothidium
angustirostratum sp. nov.

This study

- Planothidium
Planothidium brasiliense Planothidium potapovae ree e :
rostratoholarcticum Lange-
Weizel & Blanco Weitzel & Ector
Bertalot & Bak

Wetzel et al. 2019 Weitzel et al. 2019 Bak and Lange-Bertalot

2014; Wetzel et al. 2019

Elliptic-lanceolate Lanceolate, elliptic- Lanceolate to broadly Elliptical
lanceolate elliptic-lanceolate
Apices Narrowly rostrate to Rostrate Strongly rostrate Subrostrate
subcapitate

Length 17-21 um 20-28 um 11.5-14.5 um 6.4-13.5 um
Width 6.5-7.0 um 7.0-8.0 um 5.0-6.0 um 3.8-6.4 um
Central area (RV) Subcircular to rectangular Rectangular Wide rectangular Irregular
Axial area (RV) Narrow linear Narrow linear Wide rectangular Narrow, linear
Striae near to central | 3-4 shortened striae on 2-3 shortened striae on 1-3 shortened striaeon | Usually 1 shortened striae
area (RV) each side each side each side on each side
Raphe Straight Straight Straight Straight
Striae (RV) 13-14/10 um Nd Nd 14-17/10 pm
Areolae composition 3-4 rows areolae Nd 4 rows areolae 2-3 rows areolae
of striae (RV)

Axial area (SV) Narrow linear

Additional structure Cavum

(SV)

Striae (SV) 13-14/10 um
Areolae composition 3-4 rows areolae
of striae (SV)

Narrow, linear, expanded in
the central area

Cavum

15-17/10 ym
Nd

Note: “Nd” indicates no data available in the reference.

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757

Linear—lanceolate,widening
in the central area*

Cavum

12-13/10 ym

3-4 rows areolae

Narrow, straight, linear

Cavum

13-17/10 pm

2-3 rows areolae

93

Yun Li et al.: Four new Planothidium species

Planothidium maolanensis Q-M. You, P. Yu & J.P. Kociolek, sp. nov.
Figs 7,8

Holotype. SHTU! Slide GZ201510099, holotype illustrated in Fig. 7D, L. Diatom
samples are housed in the Lab of Algae and Environment, College of Life Sci-
ences, Shanghai Normal University, China.

Isotype. COLO! Material 11165, Slides are housed in the Kociolek Collection,
University of Colorado, Museum of Natural History Diatom Herbarium, Boulder,
U.S.A.

Type locality. CHINA. Maolan Nature Reserve, Libo County, Guizhou Province,
25°17'32'"N, 108°04'16'E, altitude: 650 m, collected by Kociolek J.P. & Wang
Q.X., on October 4, 2015.

Description. Light microscopy (LM) (Fig. 7A-P). Valves elliptical, with round-
ed ends not extended. Valve dimensions (n = 50): Length 9.5-18.0 um, width
5.5-7.5 um. Rapheless Valve (Fig. 7A—-H): Axial area linear-lanceolate, with
central area slightly broadened. One side of the central area has two striae with

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Figure 7. Planothidium maolanensis sp. nov. LM; A-H. Rapheless valves; I-P. Raphe valves. “=” indicates the different
valves of the same frustule. Q. SEM external view of an entire rapheless valve, scale bar as shown. Scale bars: 10 um (A-P).

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 94

Yun Li et al.: Four new Planothidium species

Figure 8. Planothidium maolanensis sp. nov SEM; A. Internal view of an entire rapheless valve. The white arrow points
to a distinctly wider virgae; B. External view of an entire raphe valve. White arrows indicate the areolae on the mantle.
C. Internal view of an entire raphe valve. Scale bar as shown.

widened spacing, opposite side striae slightly shortened. Striae weakly radiate
along the valve outline, narrowing towards axial area, 10-12 in 10 um (mea-
sured opposite hyaline area). Raphe Valve (Fig. 7I-P): Axial area linear-lance-
olate, slightly widened in the central area. Central area subcircular, bordered
by 2-3 slightly shortened striae on each side. One side of the central area has
two striae with widened spacing. Striae radiate weakly, 10-12 in 10 um. Raphe
branches straight, occasionally slightly curved.

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 95

Yun Li et al.: Four new Planothidium species

Scanning electron microscopy (SEM) (Figs 7Q, 8A-C). Rapheless Valve (Figs
7Q, 8A): Striae composed of 4-5 rows of circular areolae, markedly narrow near
the axial area, reducing to 2-3 rows of areolae, striae almost extend onto the
valve margin, and some areolae exist on the mantle. Irregular depressions pres-
ent on the axial area, central area and even on the virgae. Internally, areolae
covered. Virgae widen gradually from the ends towards the center, with striae
sunken between virgae, a distinctly wider virgae present on one side of the cen-
tral area (Fig. 8A, White arrow). Raphe Valve (Fig. 8B, C): Striae composed of
5-6 rows of small circular areolae, markedly narrowing near the central area,
reducing to 2-3 rows of areolae. striae don't extend onto the valve margin,
and some areolae exist on the mantle (Fig. 8B, White arrows). Raphe branches
straight, occasionally slightly curved. Externally, proximal raphe endings expand
into pores, distal raphe ends are curved in the same direction, and extend onto
the mantle. Internally, proximal raphe endings slightly deflected to the opposite
side, and distal raphe endings terminate in a small helictoglossa. Internally, are-
olae covered individually by membranes, striae sunken between virgae.

Etymology. The species was named for the Maolan Nature Reserve, where
the type specimen was collected.

PhycoBank registration. http://phycobank.org/105530

Distribution and ecology. This species has currently only been found at its
type locality, where it was collected from rocks in a subsurface stream and in
rapidly flowing river water (water temperature 18.0 °C, elevation 650-811 m).
The species was observed in samples GZ201510099 (3.75%), GZ201510100
(0.25%), and GZ2015108 (0.75%). In the type material (GZ201510099),
P maolanensis was rare. The associated diatom flora included Cocconeis pla-
centula (23.0%), Achnanthidium delmontii (7.5%), Navicula Iundii (7.5%), Navicula
antonii Lange-Bertalot, Navicula sp. (6.0%), Achnanthidium minutissimum (5.5%),
and other taxa with lower abundance (less than 5%) such as Platessa hustedtii,
Achnanthidium spp., Encyonema hophense Krammer, and Achnanthes prominula
Levkov & Tofilovska. Additional ecological information is presented in Table 1.

Remarks. Due to the broadly similar valve outline in LM and the absence of
cavum, Planothidium maolanensis sp. nov. can be compared with P hauckianum
(Grunow) Bukhtiyarova, P kaetherobertianum Van de Vijber & Bosak, and P. iber-
ense Rovira & Witkowski (Table 4). P maolanensis can be easily distinguished
from P. kaetherobertianum and P. iberense by valve outline and striae density,
while it shows greater overall similarity to P hauckianum. However, P maolan-
ensis can still be differentiated by the following features: (1) In PR maolanensis,
only two striae on one side of the central area show slightly increased spacing,
while the remaining striae are evenly arranged. In contrast, P hauckianum has
more striae with increased spacing on both sides of the central area. (2) The
striae of PR maolanensis consist of 4-5 rows of circular areolae on rapheless
valve and 5-6 rows on raphe valve. In P hauckianum, the striae consist of 3-4
rows of circular areolae on the rapheless valve and 2-4 rows on the raphe
valve. (3) Although the striae in P maolanensis and P hauckianum both gener-
ally narrow toward the axial area, the narrowing in P maolanensis is not gradual
or consistent. Sometimes, the middle of striae contracts, making it narrower
than the ends near the axial area. (4) P maolanensis has irregularly shaped
shallow depressions in axial area and between the striae, while P hauckianum
exhibits slit-like depressions.

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Yun Li et al.: Four new Planothidium species

Table 4. Comparison of morphological characteristics of Planothidium maolanensis sp. nov. and closely related taxa.

Reference

Valve outline
Apices

Length

Width

Central area (RV)
Axial area (RV)
Raphe

Striae (RV)

Areolae composition
of striae (RV)

Axial area (SV)

Additional structure
(SV)

Striae (SV)

Areolae composition
of striae (SV)

Planothidium maolanensis | Planothidium hauckianum

sp. nov. (Grunow) Bukhtiyarova
This study Kulas et al. 2020
Elliptical Linear-lanceolate or elliptic-
lanceolate
Rounded ends not Very weakly protracted,

extended slightly rostrate to broadly
rounded in smaller valves
9.5-18.0 um 7-30 um
§.5=7-5.0m 4.5-7.0 um
Subcircular Asymmetrical

Narrow linear Narrow, linear
Straight to slightly curved
10-12/10 um

5-6 rows areolae

Straight to weakly curved
8-10/10 um

2-4 rows areolae
Narrow linear Narrow linear
Absent Absent

10-12/10 ym

4-5 rows areolae

7-11/10 um

3-4 rows areolae

Planothidium

kaetherobertianum Van de

Vijber & Bosak

Van De Vijver and Bosak

2019

lanceolate to elliptic-
lanceolate in smaller valves
with cuneately rounded

Only very weakly protracted

apices

7-13 pm
4.0-4.5 um

Rounded to rectangular

Very narrow, linear
Straight
16-17/10 um

2-3 rows areolae

Linear to narrowly
lanceolate

Absent

15-16/10 um

2 rows areolae

Planothidium iberense
Rovira & Witkowski

Rovira et al. 2011

Ellipticallanceolate

Moderately produced,
obtusely rounded

17-26 um
6.5-9.5 um
Circular
Narrow, linear
Straight
12-15/10 pm

4 rows areolae

Narrow, linear

Absent

14-16/10 pm

3-4 rows areolae

Discussion

The four new species described in this study exhibit multiseriate striae on both
the raphe valve and the rapheless valve. On one side of the central area of the
rapheless valve, Planothidium pseudoreichardtii sp. nov. features an uncovered
depression known as a “sinus”, Planothidium liboensis sp. nov. and Planothid-
ium angustirostratum sp. nov. possess a cavum, and Planothidium maolanensis
sp. nov. lacks both sinus and cavum. The four new species were compared with
several related and morphologically similar taxa (Tables 2, 3, 4).

As acomplement to morphometry, Wetzel et al. (2019) proposed several dis-
tinguishing features in SEM, such as the pattern of valve surface smoothness
on the rapheless valve, the number of areolae per stria, and whether the striae on
the rapheless valve are interrupted towards the valve mantle. During the obser-
vation of these new Planothidium species, we found that the morphology of the
striae and the number of areolae rows per stria on the rapheless valve are rela-
tively stable within the same species. For instance, the rapheless valve striae of
P. pseudoreichardtii sp. nov. are composed of two outer rows of larger areolae
and 1-2 smaller rows in the center, with an enlarged areola present at the end
of each stria. The presence or absence of striae on the rapheless valve mantle
is also stable within the same species. Striae are present on the valve mantle
of P angustirostratum sp. nov. and P maolanensis sp. nov., whereas no striae
are observed on the valve mantle of P liboensis sp. nov. and P maolanensis sp.

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 97

Yun Li et al.: Four new Planothidium species

nov. Furthermore, we observed that the feature of valve surface smoothness on
the rapheless valve is consistent within each species. Based on the shape of
the depressions, these features can be roughly classified into three types: P. an-
gustirostratum sp. nov. exhibits circular shallow depressions on the valve sur-
face; P. liboensis sp. nov. and P maolanensis sp. nov. display irregularly shaped
depressions; while P pseudoreichardtii sp. nov. and P. rostratum, also observed
in the Xiaoqgikong Scenic Area, exhibit slit-like shallow depressions.

Currently, numerous species of the genus Planothidium have been discov-
ered in various aquatic environments and habitats worldwide (Blanco et al.
2013; N’Guessan et al. 2014; Wetzel and Ector 2014a, 2014b; Kopalova et al.
2016; Riaux-Gobin et al. 2018; Stancheva 2019; Cantonati et al. 2021; Lai et al.
2021). Additionally, some studies have investigated the ecological distribution
and physiological characteristics of Planothidium species (Sbihi et al. 2014;
Stancheva et al. 2020; Juchem et al. 2023). Despite significant advancements
over the years, the taxonomy of Planothidium is relatively well-developed, but
several questions remain open for further exploration. Future research may re-
quire additional populations or increased taxa sampling to examine the stabili-
ty of certain morphological features within the genus. Furthermore, integrating
more ecological data and molecular techniques could provide deeper insights
into the variability of these features under different environmental conditions
and their taxonomic applicability.

Acknowledgements

We would like to thank Dr. Lang Li for assistance in the preparation of literature.
We also thank the editor and reviewers for their valuable comments.

Additional information

Conflict of interest

The authors have declared that no competing interests exist.

Ethical statement

No ethical statement was reported.

Funding

This research was supported by the National Natural Science Foundation of China (Nos.
32170205 & 32100165), Natural Science Foundation of Shanghai (No. 23ZR1446800).

Author contributions

Data curation: HWZ. Formal analysis: YL. Funding acquisition: QMY. Investigation: HWZ,
QXW, PY, JPK, LXL. Methodology: QMY. Project administration: QXW, WTP. Resources:
WTP, QXW. Validation: PY, QMY, YL, LXL. Writing — original draft: YL. Writing - review and
editing: JPK, QMY.

Author ORCIDs

Yun Li © https://orcid.org/0009-0005-1251-7941
Lin-Xin Lu © https://orcid.org/0009-0004-5919-2083
Hui-Wen Zhou ® https://orcid.org/0009-0004-3926-4887

PhytoKeys 259: 81-102 (2025), DOI: 10.3897/phytokeys.259.150757 98

Yun Li et al.: Four new Planothidium species

Pan Yu © https://orcid.org/0000-0001-7937-2381

John Patrick Kociolek © https://orcid.org/0000-0001-9824-7164
Wan-Ting Pang © https://orcid.org/0000-0002-4231-4046
Quan-Xi Wang ® https://orcid.org/0000-0002-0317-6613
Qing-Min You ® https://orcid.org/0000-0002-5538-8503

Data availability

All of the data that support the findings of this study are available in the main text.

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